Method and means for depositing silicon



United States Patent 3.035 028 METHOD AND MEANS FCR DEPOSITING SILICON John E. Logan, Pittsburgh, Pa, assignor to The Wean Engineering Company, Inc, Warren, Chic, a corporation of Ohio N0 Drawing. Filed Feb. 10, 1958, Ser. No. 714,022 I 3 Claims. (Cl. 117114) 1 This invention relates to the deposition of silicon and particularly relates to the coating of metals with silicon wherein the metals to be coated are immersed in a bath comprising sodium and silicon.

It is particularly advantageous in that very short processing times are required.

This application is a continuation-in-part of my copending application, Serial Number 647,217, filed March 20, 1957, now abandoned.

A silicon coating on a base metal is advantageous in that it serves as a protective covering for the base metal beneath, protecting the base metal from corrosion, abrasion, and the like. When applied following the teaching ofmy-invention, the silicon constitutes a secure, adherent, continuous coating which may advantageously serve as a base for additional coatings and for further protective or decorative treatment. The invention may also be employed to build up a substantial depth of silicon by repetitive or sustained coatings.

I provide a bath comprising silicon and sodium. Other of the alkali metals, such as potassium or lithium., may be present, but, if present in excessive quantities, will interfere with optimum deposition of silicon on the base metal. While I prefer to employ a bath comprising silicon and sodium, it will be found that dirt, foreign matter, and the like may tend to accumulate in the bath. If such impurities accumulate to too great a degree, they may interfere with the quality of the coating, causing it to become spotted or speckled Such impurities may be skimmed from the surface of the bath from time to time to avoid undue accumulation of impurities. Preferably, I heat a bath of silicon and sodium under a protective atmosphere to some temperature less than the boiling point of sodium and then immerse a base metal in the bath. I allow the base metal to be immersed in the bath for a sufficient length of time to coat the base metal to the desired thickness with metallic silicon. Preferably, I maintain the bath at a temperature of between 1000 F. and 1300" F. and keep the base metal immersed for a period of five to fifteen seconds.

While I prefer to employ a silicon coating on ferrous products such as mild steel, it may be used to equal advantage in coating other metals. I have, for example, successfully deposited silicon coatings on such metals as copper, molybdenum, and nickel.

In the preferred practice of my invention, the bath of sodium and silicon is heated to a temperature between 1000 F. and 1300" F. The preferred bath is composed of 1% to 40% silicon with the remainder being metallic sodium. The base metal, preferably a ferrous metal such as mild steel, which is to be coated with silicon is cleaned in an alkali and then dipped in acid, rinsed, and dried. The metal which is thus cleaned is placed in the bath and withdrawn. A cover is provided over the bath to maintain the protective atmosphere intact, and means are pro vided to permit the work to be inserted into and removed from the bath. In the case of steel strip, for example, a long thin opening with a pair of opposed flexible wipers may be provided along the edges of the opening to hold the atmosphere intact while the strip is entering and leaving the bath. Upon withdrawal of the work, it will be found that the silicon is coated upon the base metal but that substantially all of the sodium remains in the pot in which the heated bath is contained. The silicon is thus withdrawn from the bath but the sodium remains in acid cleaning, rinsing with water and drying.

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place, thus decreasing the silicon concentration. Consequently, it is necessary to add silicon to the bath at regular intervals, but the sodium content of the bath remains substantially constant, subject to minor losses. In some instances, darker spots of silicon have been noted upon the coated strip, but they do not appear to be harmful or deleterious in any way and seemingly result from extended immersion at higher temperatures and higher silicon concentrations.

It is obvious that my invention may be employed equally well for the processing of a continuous flow of material, such as' continuous steel strip, or it may be used to coat discrete articles such as steel sheets, castings and the like. The relatively high speed of coating makes the invention particularly advantageous for continuous processes. The heated bath may also serve the additional purpose of adding heat to the work being treated. It has also been observed that in the case of prolonged or repeated exposures to the bath, the layer of siliconon the workpiece will continue to build up. Thus, the invention may be advantageously used to form a silicon deposit of appreciable thickness. :For example, the deposition might be made on a silicon base in the first instance.

A few examples will serve to illustrate a preferred form of my invention. Ordinary low carbon steel strip was prepared by alkaline cleaning to remove oil and grease, It was then inserted through a protective atmosphere into a heated pot containing sodium and silicon. The bath was prepared by reducing a commercially obtainable grade of silicon to powder form and heating it with a commercial grade of sodium. A cover was provided over the pot having a sleeve through which the strip was inserted and a slight positive pressure of the inert gas maintained thereunder. Upon Withdrawal, the strip was allowed to cool in open air, and the following results were noted:

Silicon Immersion Concen- Temperature Time, tration of Result seconds the bath,

percent 840 F 60 10 Continuous, regular coating. 1,100 F 15 1 Do.

,250 F 15 10 Continuous, adherent coating with some darker spots.

As the temperature of the bath is increased, the speed of the coating generally increases. Increased time will likewise increase the depth of the coating. Several examples will serve for illustration. The following tests were made under the same general conditions as described above:

It should be noted that as the temperature approaches the boiling point of the sodium, excess sodium fumes will be encountered, and advantages resulting from faster coating may be offset by the increased difiiculty of handling the fumes. It is possible, on the other hand, to reduce the temperature at least as low as 840 F. and obtain satisfactory coatings if the immersion time is long enough and the bath is agitated while the metal is so immersed.

If the concentration of silicon in the sodium is reduced to a very low figure, the immersion time is increased to get a continuous coating over the strip. For this reason is is preferred to maintain a silicon concentration of at least 3%, but a good coating can be secured at lower concentrations by increasing the immersion time, or by agitating the bath during immersion of the strip, or by a combination of both. The following examples are illustrative of the effect of concentration of silicon on the coating:

It is believed that when the silicon coats the steel, an iron-silicon alloy is formed at the surface of the strip and that there must be a sufiicient exposure of the strip to the silicon at elevated temperatures to form such an alloy layer before good adhesion will be obtained. It has been observed that in some cases where the immersion time is brief or the temperature is too low, the silicon coating may be peeled off by bending or flexing the metal. The invention is not predicated on this thecry, and it is expressed only as a matter of belief, the exact mechanism being unknown. As previously mentioned, however, the silicon will build up upon silicon, forming a growth of silicon which may be used where discrete silicon particles or a crystalline silicon structure is desired.

It has been further observed that if the concentration of silicon in the sodium exceeds approximately 20%, there is a tendency for some metallic sodium to be entrained in the silicon coating and withdrawn from the bath with the workpiece. Such entrainment is not desirable for the reason that the sodium will later react With water. The reaction causes gassing and may result in the coating being marked and of irregular thickness. It is desirable, therefore, to maintain the concentration of silicon in the bath below approximately 20%. Within the range of normal working temperatures, however, the silicon concentration may be increased to about except for the possibility of entrained sodium. At concentrations of more than 40% silicon, however, the silicon tends to accumulate in the bottom of the pot as an agglomerated mass, and the strip in this part of the bath will not be coated. It will be understood, of course, that agitation of the bath, its temperature, and the quality of coating required will all affect the foregoing. It is clear that the processing times required are quite short and that the invention may readily be used in high output environments.

While I have described certain present preferred embodiments of my invention, I do not limit myself thereto, and my invention may be otherwise variously practiced within the scope of the following claims.

I. claim:

1. A liquid bath for coating a ferrous metal with silicon, said bath being between 3% silicon and 20% silicon with the balance being substantially metallic sodium.

2. A bath for coating a ferrous metal with silicon, said bath being between 3% silicon and 20% silicon with the balance being sodium, the bath being heated to a temperature between 1000 F. and 1300 F.

3. The method of coating a ferrous metal with silicon which comprises cleaning and washing the ferrous metal, immersing it in a mixture comprising 1% to 40% silicon, the balance being substantially liquid metallic sodium, and protecting the mixture with an inert atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 1,073,560 Allen Sept. 16, 1913 1,869,494 Osborg Aug. 2, 1932 2,097,024 Enders Oct. 26, 1937 2,431,326 Heynoth Nov. 25, 1947 2,438,892 Becker Apr. 6, 1948 2,509,875 McDonald May 30, 1950 2,665,998 Campbell et al. Jan. 12, 1954 2,703,296 Teal Mar. 1, 1955 2,848,352 Noland et a1. Aug. 19, 1958 OTHER REFERENCES Hansen: Constitution of Binary Alloys, page 1004, 2nd Ed, 1958, 

3. THE METHOD OF COATING OF FERROUS METAL WITH SILICON WHICH COMPRISES CLEANING AND WASHING THE FERROUS METAL, IMMERSING IT IN A MIXTURE COMPRISING 1% TO 40% SILICON, THE BALANCE BEING SUBSTANTIALLY LIQUID METALLIC SODIUM AND PROTECTING THE MIXTURE WITH AN INERT ATMOSPHERE. 